I think it depends. There have been a couple of team car camera and mic episodes. In one, Johan of Disco is giving instructions in English and then repeats himself in Spanish. In another, I think it was CSC, they were switching between English, French, and German.

Cyclists of the world, unite! You have nothing to lube but your chains!

Actually, english became the official language that teams communicate with, this info is repeated every stage on VS. However, whenever you have a rider that doesn't speak english than you try whatever you need to pass the message.